1. Field of the Invention
The present disclosure relates to a semiconductor technique. More specifically, the present disclosure relates to a lateral double diffused metal oxide semiconductor (LDMOS) transistor to realize uniform threshold voltage.
2. Discussion of the Related Art
The improvement in integration level of semiconductor devices, and thus development in fabrication design techniques thereof, has brought about a number of efforts to realize a semiconductor system on a single chip. Such a single chip system has been developed, based on technology to integrate controllers, memories and circuits operating at low voltages onto a single chip.
However, to realize low weight and miniaturization of a semiconductor system, it is necessary to integrate a circuit to control power of the system (e.g., input and output terminals and main functional circuits) onto a single chip. However, input and output terminals generally cannot be realized as a low-voltage CMOS circuit, because high voltages may be applied thereto. Accordingly, these terminals may be generally realized by high-voltage power transistors.
To reduce the size and weight of systems, input/output terminals of a power source and a controller should be integrated into a single chip. A technology to realize this integration is a power IC, wherein a high-voltage transistor circuit and a low-voltage CMOS transistor circuit are integrated into a single chip.
Such a power IC technology is an improvement of a vertical DMOS (VDMOS) device, a related art discrete power transistor, which may realize a lateral DMOS (LDMOS) device. In a LDMOS device, the drain is laterally arranged to allow current to laterally flow and the drift region is interposed between a channel and the drain to assure high voltage breakdown.
However, in accordance with a general LDMOS technology, photoresist used to form p-type body regions typically undergoes a variation in its profile when ions are implanted. That is, as shown in FIG. 1, after ion-implantation to form p-type body regions, the profiles of the photoresist are varied, thus causing unbalance between left and right photoresist profiles, which results in non-uniformity between channel lengths, leading to deterioration in transistor characteristics. This non-uniformity between left and right channel lengths causes a transistor with the shorter channel to turn on earlier, as compared to another transistor with a longer channel. This means that the transistor with the shorter channel has a lower threshold voltage than a target threshold voltage, thus causing a non-uniform threshold voltage across different devices.